Protective coating for a magneto-optical disc

ABSTRACT

A magneto-optical disc includes protective overcoats that significantly improve the durability of the disc. In particular, the protective overcoat involves a carbon layer over the magnetic, optical data storage layer. The magnetic, optical data storage layer includes a magnetic metal or alloy with a Curie temperature accessible by optical heating. In preferred embodiments, the carbon coating has a small absorption coefficient for selected optical frequencies.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority from U.S. ProvisionalApplication Serial No. 60/090,480 filed on Jun. 24, 1998, entitled“DESIGN OF HYBRID OVERCOAT FOR MAGNETO-OPTICAL RECORDING MEDIA,”incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The invention relates to magneto-optical disc having an improvedsurface coating. More particularly, the invention relates to amagneto-optical disc with a protective overcoat that improves thetribological properties and reduces wear.

[0003] A variety of media can be used for electronic data storage. Inparticular, magnetic discs have been used extensively for high densitydata storage. In addition, optical data storage disc systems are usefulfor storing large quantities of data. The data is accessed by focusing alight beam, generally a laser beam, onto a data surface of a disc anddetecting light reflected from or transmitted through the data surface.

[0004] Generally, in optical data storage systems, data is in the formof physical or magnetic marks carried on the surface of the disc. Thephysical or magnetic marks are detected using the laser light. Phasechange and magneto-optical systems provide erasable optical discsystems, which are clearly desirable for certain applications.Magneto-optical systems read data by measuring the rotation of theincident light polarization due to reflection from a magnetic materialwithin the storage medium.

[0005] A magneto-optical disc includes a recording layer of a magneticmaterial. The magnetic material in the recording layer can be magnetizedin an oriented direction. The data generally is stored in concentrictracks.

[0006] To read data from the disc, an optical pickup device is used. Theoptical pickup device irradiates the disc with a low power light beam,generally from a laser, and detects the reflected light. Changes in thepolarization of the light due to the Kerr effect can be measured in thereflected light. Using the change in polarization, the magnetization ofthe point on the recording layer can be measured.

[0007] To record/write or erase information, the recording layer isheated to a temperature above the Curie temperature by irradiating thematerial with a more intense light beam, generally from a laser. Themagnetization of the heated point on the recording layer can be changedwith an external magnetic field. To write data on the disc, a magnetichead is used for supplying the external magnetic field. The same magnetor a separate magnet can be used to erase or initialize the disc.

[0008] When only a single magnetic layer is used, it is required toinitialize the magnetic layer by aligning the magnetization of themagnetic layer in one direction before writing data. This process can besimplified by using multiple magnetic layers. Two, three or moremagnetic layers can be used to facilitate the reading and writingoperations. With any of the magneto-optical disc structures, animportant design consideration is that light must be able to focus on amagnetic data storage layer having an appropriately selected Curietemperature.

SUMMARY OF THE INVENTION

[0009] In a first aspect, the invention pertains to a magneto-opticalstorage medium comprising:

[0010] a data storage means for the optical encoding of data; and

[0011] protection means for improving the durability of themagneto-optical storage media.

[0012] In another aspect, the invention pertains to a magneto-opticalstorage medium comprising:

[0013] a non-magnetic substrate;

[0014] a magnetic layer over the non-magnetic substrate, the magneticlayer comprising a magnetic metal or alloy having a Curie temperatureaccessible by optical heating;

[0015] a carbon layer over the magnetic layer.

[0016] In a further aspect, the invention pertains to a method ofproducing a magneto-optical disc comprising depositing a carbon layeronto a disc with a magnetic layer comprising a magnetic metal or alloyhaving a Curie temperature accessible by optical heating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic, fragmentary sectional view of a magneticdisc with an improved overcoat.

[0018]FIG. 2 is a schematic, fragmentary sectional view of an embodimentof an undercoat.

[0019]FIG. 3 is a schematic, fragmentary, sectional view of anembodiment of a magnetic layer.

[0020]FIG. 4 is a schematic, fragmentary sectional view of a preferredembodiment of a magnetic layer.

[0021]FIG. 5 is a schematic, fragmentary, sectional view of a preferredembodiment of a magnetic disc with an improved overcoat.

[0022]FIG. 6 is a schematic, fragmentary sectional view of a magneticdisc with a magnetic recording layer on each surface of the substrate.

[0023]FIG. 7 is a schematic, fragmentary, sectional view of analternative preferred embodiment of a magnetic disc with an improvedovercoat having a magnetic layer on both sides of the disc.

[0024]FIG. 8 is a top view of a cartridge holding a magnetic opticaldisc.

[0025]FIG. 9 is a cut away, perspective view of a hard disc driveincluding a magnetic optical disc.

[0026]FIG. 10 is a schematic view of a data storage device for use withan magneto-optical disc.

[0027]FIG. 11A is a plot of reflectance as a function of lightwavelength for a magneto-optical disc with a structure essentially asshown in FIG. 5 with a hydrogenated carbon film.

[0028]FIG. 11B is a plot of index of refraction and the absorptioncoefficient obtained with the optical disc used to produce the plot ofFIG. 11A.

[0029]FIG. 12A is a plot of reflectance as a function of lightwavelength for a magneto-optical disc with a structure essentially asshown in FIG. 5 with a nitrogenated carbon film.

[0030]FIG. 12B is a plot of index of refraction and the absorptioncoefficient obtained with the optical disc used to produce the plot ofFIG. 12A.

[0031]FIG. 13A is a plot of reflectance as a function of lightwavelength for a magneto-optical disc with a structure essentially asshown in FIG. 5 with a hydro-nitrogenated carbon film.

[0032]FIG. 13B is a plot of index of refraction and the absorptioncoefficient obtained with the optical disc used to produce the plot ofFIG. 13A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] It has been discovered that a thin protective layer can be placedover the surface or a portion of the surface of a magneto-optical discto improve the tribological properties of the disc. The protective layercan be used without interfering with the optical properties required forthe function of the disc. In particular, a thin carbon layer,optionally, topped with a thin lubricant layer provides both additionalprotection for the functional layers and an improved hydrodynamicinterface. Thus, the fly height of heads used for reading and/or writingon the disc can be reduced without resulting in an undesirably shortlifetime of the disc due to interactions between the head and the discsurface.

[0034] A design requirement for magneto-optical disc is that focusedlight must be able to reach and thereby heat a magnetic layer. Toperform the magnetization and optical functions, one or moresliders/heads functioning as an optical head and/or magnetic head mustfly over the spinning disc during operation. To increase storagedensities, fly heights can be decreased such that the head flies closerto the disc surface. Decreasing fly heights impose severe requirementson the tribological performance of both the disc and the head.Approaches for improving the hydrodynamic properties of the disc surfacecannot interfere significantly with the optical and/or other propertiesof the disc surface. At the same time, the disc must be durable. Contactbetween the disc surface and a read/write head can damage the discand/or induce wear on the disc surface.

[0035] Conventional overcoats for the magneto-optical media include adielectric ceramic material, such as SiN_(x), AlN_(x), AlO_(x) and/orSiO_(x), with a thickness from about 700 angstroms (Å) to about 1200 Å.To increase the durability and improve the hydrodynamic character, athin carbon layer is placed over the dielectric ceramic layer and a thinlubricant layer optionally is placed over the carbon layer. The carbonlayer and the lubricant layer significantly increase the durability andimprove the tribological properties of the disc-head interface. Inparticular, for one design examined durability was improved such that auncoated disc which suffered significant damage in a single cycle of aContact-Start-Stop test to being able to survive thousands of cycles ofthe test. For designs in which the light must focus through the topsurface of the disc, the protective overcoat layers preferably do notinterfere significantly with the optical properties of the disc.

[0036] The magnetic layer on the disc includes at least one layer of amagnetic material that has a Curie temperature that can be reached byirradiation with a focused laser beam. Thus, irradiation of a spot ofthe magnetic material can be used to write and/or erase data at thatpoint. Additional magnetic coats with different properties can beincluded to facilitate the reading, writing and/or erasing operations.The magnetic materials generally include metal alloys with desiredmagnetic field strengths and Curie temperatures.

[0037] Referring to FIG. 1, a fragmentary cross sectional view of thegeneral structure of an improved disc 100 is depicted schematically.Improved disc 100 includes a substrate 102, an undercoat 104, a magneticlayer 106, a dielectric overcoat 108, a carbon overcoat 110 and, inpreferred embodiments, a lubricant overcoat 112. Each functional layer104-112 can include multiple physical layers. As described furtherbelow, functional layers 104-112 can be repeated in their mirror imageon bottom surface 114 of substrate 102.

[0038] Substrate 102 forms a majority of the disc bulk. Thus, substrate102 supports the functional layers for data storage. Substrate 102 canbe formed, for example, from an organic polymer material, metal, glass,a ceramic material or a combination thereof. Preferred metals includealuminum or aluminum alloy plated with NiP.

[0039] In one approach to the use of a magneto-optical disc, the lightis transmitted through the substrate to the magnetic layer. The opticalcomponents for focusing the light onto the disc surface are located onthe side of the magneto-optical disc opposite the magnetic layer whilean external magnet or magnets for altering the magnetic orientation ofthe magnetic layer are located on the side of the disc near the magneticlayers. In alternative embodiments, the optical components and theexternal magnet(s) are both placed on the side of the disc adjacent themagnetic layer. If the optics are placed on the side of the discadjacent magnetic layer 106, substrate 102 and undercoat 104 do not needto be transparent to light.

[0040] Undercoat 104 generally includes one or more dielectric layers.Suitable materials for the formation of the dielectric layer include,for example, SiN_(x), SiO_(x), AlN_(x), AlO_(x) or combinations thereof.The dielectric layers make good water vapor free layers to protect themagnetic layer from corrosion. The dielectric layer preferably has athickness from about 50 Å to about 100Å.

[0041] If undercoat 104 does not need to be transparent, undercoat 104can also include a layer of aluminum or aluminum alloy, such as AlCr,AlTi, AlCu, or AlMo. The aluminum or aluminum alloy layer can functionas a heat sink and reflective layer. Having a heat sink provides bettercontrol of the heating to erase or overwrite the magnetic layer andreduction in noise in the measurement of reflected light from magneticlayer 106. Increasing reflection also improves the signal-to-noiseratio, i.e., carrier-to-noise ratio, of the measurements since thesignal results from the reflected light.

[0042] The aluminum or aluminum alloy layer can be located adjacent adielectric layer or between two dielectric layers. A preferredembodiment of an undercoat layer 120 with an aluminum or aluminum alloylayer 122 between two dielectric layers 124, 126 is depicted in FIG. 2.Dielectric layer 124 can help to protect magnetic layer 106 and layer122 from corrosion. Similarly, dielectric layer 126 keeps moisture awayfrom substrate 102 to reduce corrosion and film adhesion problems. Inaddition, dielectric layer 124 can improve optical recording and reducethe servo read out modulation signal to facilitate oriention of aread/write element or slider along the disc surface.

[0043] Magnetic layer 106 can include one or more layers offerromagnetic alloy. Preferred ferromagnetic alloys are formed from oneor more rare earth elements and one or more transition metals. Examplesof suitable ferromagnetic alloys include, for example, TbFeCo, GdFeCo,TbFeCoZr, DyFeCo, and GdDyFeCo. The particular composition of the alloyis selected to obtain a desired Curie temperature and magneticcoercivity.

[0044] In the simplest embodiments, magnetic layer 106 includes a singlelayer of ferromagnetic alloy. The ferromagnetic alloy has a Curietemperature that can be reached by shining a relatively intense laserbeam at a spot on the ferromagnetic alloy. Similarly, the Curietemperature must be significantly higher than the operating temperatureof the disc drive such that thermal effects do not result in data loss.Reasonable ranges for the Curie temperature are from about 250° C. toabout 350° C., and preferably near 300° C.

[0045] The Curie temperature (T_(c)) is a phase transition temperatureabove which the magnetization of a ferromagnetic vanishes and thematerial becomes paramagnetic. When the temperature of the materialcools below the Curie temperature in the presence of an externalmagnetic field, the magnetization of the resulting ferromagnetic isoriented according to the external field.

[0046] Magnetization can be used for data storage because thepolarization of light striking the ferromagnetic material will beeffected by the orientation of the magnetic field. This is termed theKerr effect. In particular, circularly polarized light will be effectedupon reflection from a magnetic material having a component, of themagnetization parallel or anti-parallel to the direction of impingingradiation. The effect of the ferromagnetic material on the polarizationof the reflected is different if the magnetic component is parallel oranti-parallel to the incident direction of the impinging radiation. Thischange in polarization can be measured. Therefore, retrievableinformation can be stored based on the orientation of the magnetizationat a point along the magnetic (magneto-optical) layer.

[0047] The use of a single ferromagnetic layer is found to providemeasurements with a low carrier-to-noise ratio and difficulties withrespect to reinitializing the magnetization in order to overwrite thedata. A variety of approaches using multiple ferromagnetic layers havebeen used to improve the performance properties of magneto-optic storagematerials. These multiple layer structures generally include a memory ordata storage layer with a relatively low Curie temperature and areference layer with a relatively high Curie temperature. A twoferromagnetic layer structure 130 for the magnetic layer is shownschematically in FIG. 3. The ferromagnetic reference layer 132 isadjacent ferromagnetic memory layer 134. An optional spacer layer 136 isshowin between ferromagnetic layers 132, 134. Spacer layer 136 can beanother ferromagnetic layer or a dielectric layer.

[0048] A low field magnetic is used for writing/erasing memory layer 132and a high field magnetic is used for initializing reference layer 130.The intensity of the laser beam can be varied to alternatively effectboth the reference and memory layers 130, 132 or just memory layer 130.If the Curie temperature of reference layer 130 is high enough, themagnetization of reference layer 130 can be established once duringfabrication such that its magnetization remains unchanged during use.

[0049] Several multiple ferromagnetic layer structures are described inU.S. Pat. No. 5,361,248 to Hatwar et al., entitled “Direct OverwriteMagneto-Optical Storage Medium Not Requiring an Initialization Magnet,”incorporated herein by reference. In addition, magneto-optical discswith three ferromagnetic layer structures are described in U.S. Pat. No.5,615,180 to Mieda at el., entitled “Magneto-Optical Recording Mediumand Magneto-Optical Recording Apparatus Capable of Performing aLight-Modulation Overwriting Operation, and U.S. Pat. No. 5,665,467 toNakayama et al., entitled “Magneto-Optical Recording Medium,” both ofwhich are incorporated herein by reference.

[0050] In a particularly preferred embodiment of the magnetic layer,shown in FIG. 4, magnetic layer 140 has a write assist layer 142, arecording layer 144, an auxiliary layer 146 and a magnetic readout layer148. Write assist layer 142 preferably comprises a magnetic rareearth/transition metal alloy, such as TbFe, TbFeCo or FeCoX, where X isDy, Gd or Sm, and preferably has a thickness of about 10 nm (100Å).Write assist layer 142 preferably has a Curie temperature (T_(c)) ofabout 250° C. Write assist layer 142 provides stability for the use ofhigher data storage density on the disc.

[0051] Recording 144 can comprises a rare earth/transition metal alloy,such as TbFe, TbFeCo, TbFeCoX, DyFeCoX and the like, where X is Al, Y orNd. Recording layer 144 has a high Curie temperature, perpendicularanisotropy and thermal-magnetic features, in particular, a coercivity(H_(c)) and magnetic moment that preferably are a function oftemperature. Recording layer 144 preferably has a thickness from about25 nm to about 40 nm. Recording layer 144 is used for writing data tothe disc.

[0052] Auxiliary layer 146 can be a dielectric material, such as a watervapor free material, which can comprise SiN_(x), AlN_(x), SiO_(x) orAlO_(x). Alternatively, auxiliary layer 146 can be a rare earthtransition metal alloy with a low Curie temperature and a highcoercivity. Auxiliary layer 146 preferably has a thickness of about 10nm.

[0053] Magnetic readout layer 148 is formed from a rare earth/transitionmetal alloy, such as GdFeCo, GdFeCoX or GdFeCoXY, where X is Al, Y, orNd, and Y is Cr, Ta, or Nb. Magnetic readout layer 148 is particularlypreferred for use in a magneto-optical disc designed for shining lightfrom the top surface rather than through the substrate. Such aconfiguration for the magneto-optical disc is called first surfacerecording and is described further below. Data stored on recording layer144 is copied onto readout layer 148 for reading the data. Auxiliarylayer 146 facilitates the data transfer from recording layer 144 toreadout layer 148

[0054] A preferred embodiment of the structure in FIG. 4 is summarizedin Table 1. H_(c), Oe in perpendic. Layer T_(c) direction Thickness 142GdFeCo 250° C. <500 10 nm 144 TbFeCo 250-350° C. >10,000 25-40 nm 146TbFeCoAl 120° C. >10,000 10 nm 148 GdFeCo >300° C. <3000 25-30 nm

[0055] The carbon layer preferably includes amorphous hydrogenatedcarbon, amorphous nitrogenated carbon and/or amorphoushydro-nitrogenated carbon. In some preferred embodiments, the carbonlayer includes from about 10 to about 40 molar percent hydrogen and morepreferably from about 20 to about 30 molar percent hydrogen. In somepreferred embodiments, the carbon layer includes from about 5 to about30 molar percent nitrogen. Preferred embodiments of a hydro-nitrogenatedcarbon layer include from about 3 to about 10 molar percent nitrogen andfrom about 15 to about 30 molar percent hydrogen. Hydrogenated carbon ispreferred because it has a lower absorption coefficient for lightwavelengths from about 400 nm to about 700 nm. In preferred embodiments,the carbon layer has a thickness greater than about 25 Å, preferablyfrom about 25 Å to about 50 Å, and more preferably from about 25 Å toabout 38 Å.

[0056] The carbon coating can have a profound impact on the durabilityof the disc. In particular, a magneto-optical disc without a carboncoating obtains a wear mark during a single cycle of Contact-Start-Stop(CSS) testing. During a CSS test cycle, the magnetic head begins incontact with the disc. Then, the disc is accelerated to a selectedrotational speed. After maintaining this rotational speed for a shortperiod of time, the disc is stopped such that the head comes intocontact with the disc again. With the addition of a carbon coating, thedisc preferably can endure more than a 1000 cycles, more preferably morethan about 1500 cycles and even more preferably more than about 2000cycles of a CSS tester. With the further addition of a lubricant layer,the disc preferably can endure more than 20,000 cycles with ahydrogenated carbon coating.

[0057] For magneto-optical disc structures with the light illuminatedthrough the top surface, the carbon layer preferably is highlytransmitting of the light frequency used. The light wavelength is about660 nm for a red laser and about 410 nm for a blue laser. In particular,the carbon layer preferably has an absorption coefficient at both 660 nmand 410 nm of less than about 1.0, preferably less than about 0.5, morepreferably less than about 0.15 and more preferably less than about 0.1.It is especially preferred for the carbon layer to have an absorptioncoefficient of about 0. Hydrogenated carbon has a significantly smallerabsorption coefficient for a given coating thickness compared withnitrogenated carbon or hydro-nitrogenated carbon. Different types ofcarbon coatings generally have similar reflectivities. Thus, the carboncoating greatly improves the durability of the magneto-optical mediawithout changing significantly the carrier-to-noise ratio (CNR).

[0058] Optional lubricant overcoat 112 comprises a polymeric material.Lubricant overcoat 112 preferably has a thickness less than about 35 Å,preferably less than about 30 Å, more preferably between about 15 Å andabout 25 Å.

[0059] Preferred polymers include, for example, fluorinated polymers,such as perfluoropolyethers and derivatives thereof. Suitableunsubstituted perfluoropolyethers polymers include, for example,Fomblin® Z-60 (average molecular weight (AMW)=about 60,000 atomic massunits (AMU) or Daltons), Fomblin® Z-25 (AMW=about 25,000 AMU) andFomblin® Z-15 (AMW=about 15,000 AMU). The Fomblin® unsubstitutedperfluoropolyethers made by Montedison (Ausimont) S.P.A., Milan, Italyhave molecular formulas of CF₃O(CF₂CF₂O)_(n)(CF₂O)_(m)CF₃, where n and mvary to yield particular products with average molecular weights ofspecified values.

[0060] Suitable fluorinated polyethers include perfluoropolyethers withfunctional end groups. Suitable difunctional perfluoropolyethersinclude, for example, Fomblin® Z-DOL (hydroxyl end groups), Fomblin®AM2001 (piperonyl end groups), and Fomblin® Z-DISOC (isocyanate endgroups). Fluorinated polymers with functional end groups may help thelubricant overcoat bind to a carbon substrate. Other suitableperfluoropolyethers are available under the tradenames Demnum® fromDaikin Kogyo Co., Japan and Krytox® from DuPont, Wilmington, Del.,having a basic molecular formula of F(CF₂CF₂CF₂)_(n)CF₂CF₃.

[0061] Preferred embodiments have been described for the layers shown inFIG. 1. The various preferred layers can be combined in the formation ofa particularly preferred magneto-optical disc structure 160, as shown inFIG. 5. Magneto-optical disc 160 includes a substrate 162, an undercoat164, a magnetic layer 166 and a composite overcoat 168. Undercoat 164includes a first dielectric layer 170, a reflective/heat sink layer 172and a second dielectric layer 174, where the heat sink layer 172 isbetween dielectric layers 170, 174. Magnetic layer 166 includes a writeassist layer 176, a recording layer 178, an auxiliary layer 180 and amagnetic readout layer 182. Composite overcoat 168 includes a dielectriclayer 184, carbon layer 186 and lubricant overcoat 188.

[0062] As noted above, both surfaces of the substrate can each include aset of functional, i.e., magnetic layers. Structure on either side ofthe substrate may or may not be identical mirror images of each other.Generally, in preferred embodiments, the layers on either side of thesubstrate are mirror images of each other, such that the magneto-opticaldisc is approximately symmetric in the plane passing through the centerof the substrate, or equivalently the disc. One approach to theformation of a magneto-optical disc with magnetic layers along bothsurfaces involves gluing together two discs with single magnetic layers,such as shown in FIG. 1. Alternatively, appropriate films can be coatedonto boht sides of a substrate. A magneto-optical disc 200 withsymmetric magnetic layers relative to substrate 202 is shown in FIG. 6.Disc 200 includes undercoats 204, magnetic layers 206, dielectricovercoats 208, carbon overcoats 210 and optional lubricant overcoats212.

[0063] In a particularly preferred embodiment, the structure shownadjacent the substrate in FIG. 5 is repeated on the other side of thesubstrate. Such a preferred embodiment of a magneto-optical disc 220 isshown in FIG. 7. Substrate 222 is located at the center of disc 220. Anundercoat layer 224, magnetic layer 226 and composite overcoat 228 arelocated at the top surface of substrate 222. Layers 224, 226 and 228have structures corresponding respectively to layers 164, 166 and 168 ofFIG. 5. Undercoat layer 230, magnetic layer 232 and composite overcoatlayer 234 are respectively the mirror images of layers 224, 226 and 228.

[0064] Generally, the undercoat layer or layers and the layer or layersof the magnetic layer can be deposited by vapor deposition approaches,such as sputtering. Various forms of sputtering can be used, such asfacing target sputtering, DC magnetron sputtering, RF magnetronsputtering, DC diode sputtering, RF diode sputtering, or physical vapordeposition sputtering. Other vapor phase approaches can be used todeposit the layers such as chemical vapor deposition, ion implantation,plasma spraying, plasma enhanced chemical vapor deposition, thermallyassisted evaporation, and electron beam assisted vapor deposition. Thedielectric layers can be deposited by performing the sputtering or otherdeposition approach with hydrogen and/or nitrogen generally diluted withargon or other inert gas in the atmosphere surrounding the surface.

[0065] The dielectric layer and carbon layer of the composite overcoatcan be deposited using these vapor phase deposition approaches. Thestructure of the carbon layer generally depends on the depositionapproach. In particular, the carbon layer can be, for example,amorphous, graphitic, diamond-like-carbon, or a mixture thereof. Thecarbon layer generally is deposited with hydrogen and/or nitrogen in theatmosphere surrounding the substrate. To form the nitrogen or hydrogendoped carbon coatings, the sputtering is performed in the presence ofabout 5 percent to about 30 percent nitrogen or hydrogen in argon. Alubricant overcoat can be applied by dipping, buffing, brushing, spincoating, and the like. The concentration of lubricant in the lubricantlayer generally is in the range of 0.2% to about 0.3% with the balancebeing solvent, such as C₆F₁₄, C₄F₉OCH₃, C₄F₉OC₂H₅, C₅H₂F₁₀, or mixturesthereof.

[0066] A relatively weak light source is used to read the stored data.Preferred light sources include a laser with an intensity of about 3 to4 milliwatts (mW). To write onto the disc, a stronger light source, suchas a 10 mW laser is used to heat the read layer above its Curietemperature. A write layer assists with the initialization of the readlayer prior to setting the orientation of the magnetic moment in theread layer using a magnet in a write head. To perform first surfacerecording, the write head preferably includes optics for directing lightonto the disc for reading and/or writing. As noted above, a write assistlayer can be used to stabilize the magnetization of all the magneticlayers, which can be especially helpful as the storage densityincreases.

[0067] For distribution, a magneto-optical disc can be assembled into acartridge 250, as shown schematically in FIG. 8. In this embodiment,cartridge 250 includes slide 252 that moves along track 252, such thatslide 252 can be moved to expose a portion of magneto-optical disc 256to the exterior of cartridge 250. Thus, with slide 252 moveappropriately, a head and/or other magnetic or optical components can bebrought into the proximity of the surface of disc 256. Cartridge 250 caninclude a magnet 258. Magnet 258 can be used to initialize a write layeror other similar magnetic layer. Magnet 258 is moved into place whenneeded. Magnet 258 can be used effectively especially in embodimentswhere light is directed through the substrate.

[0068] Alternatively, a magneto-optical disc can be part of a hard discdrive. Suitable hard disc drives include Optically Assisted WinchesterDrives and Optically Assisted Writing Drives. A hard drive 262 is shownschematically in a cut away view in FIG. 9. A magneto-optical disc 264is attached on a drive motor 266.

[0069] A magneto-optical disc is used in conjunction with a disc drivesystem for reading/writing, such as shown schematically in FIG. 9. Thedisc drive can be a hard disc drive, a disc drive that interfaces with adisc cartridge, or the like. Disc drive 270 includes a disc drive 272that supports and rotates magneto-optical disc 274. Actuator 276controls arm 278 such that head 280 at the end of arm 278 is positionedover a desired portion of disc 274. Actuator 276 can move arm 278 byrotation or lateral motion. The embodiment shown involves first surfacereading where light is directed from light source 282 to the top surfaceof disc 274. Light can be directed along optical fiber 284 or othersuitable optical components. Read/write unit generally is interfacedwith a computer processor 286. If desired, the disc drive can bedesigned for use with cartridges containing magneto-optical discs suchas the cartridge shown in FIG. 8.

[0070] The optical properties were examined for three different carbonfilms. The three carbon films were, respectively, a hydrogenated carbonfilm, a nitrogenated carbon film and a hydro-nitrogenated film. Thecarbon films were deposited by AC reactive sputtering using an AC powersupply operating in the frequency range from about 40 KHz to about 400KHz to a thickness from about 25 to 50 Å. The hydrogenated carbonincluded about 25 mole percent hydrogen. The nitrogenated film includedabout 15-20 mole percent nitrogen. The hydro-nitrogenated carbonincluded about 5 mole percent nitrogen and about 20 mole percenthydrogen.

[0071] For each film the reflectivity, index of refraction andabsorption coefficient were measured. The results are plotted in FIG. 10for the hydrogenated carbon film, FIG. 11 for the nitrogenated carbonfilm and FIG. 12 for the hydro-nitrogenated carbon film. Thereflectivity is plotted in the A panels of FIGS. 10-12 while the indexof refraction and the absorption coefficient are plotted in the B panelof FIGS. 10-12. The reflectivity is roughly the same for the discs withthe three carbon films. The reflectivity increases from 190 nm to about800 nm and reaches a plateau from about 800 to about 900 nm.

[0072] For the magneto-optical disc with the hydrogenated carboncoating, the index of refraction gradually increases to about 500 nm andthen slowly decreases as a function of wavelength. For themagneto-optical disc with the nitrogenated carbon coating, the index ofrefraction increases over the whole frequency range from 190 nm to about900 nm. Similarly, for the magneto-optical disc with thehydro-nitrogenated carbon coating, the index of refraction increasesover the whole frequency range from 190 nm to 900 nm.

[0073] For the magneto-optical disc with the hydrogenated carboncoating, the absorption coefficient increases slowly from 190 nm toabout 300 nm and reaches a plateau from about 300 nm to about 350 nm ata maximum value of about 0.25. At higher wavelengths, the absorptiondecreases slowly from about 350 nm to about 800 nm where another plateauis reached. The absorption coefficient reaches a minimum value of about0.02. By increasing the hydrogen content of the carbon film to valuesfrom about 30-35 mole percent hydrogen, it is possible to get anabsorption coefficient of about 0 with 660 nm light.

[0074] The plots of the absorption coefficients for the magneto-opticaldiscs with the nitrogenated carbon coating and hydro-nitrogenated carboncoating are similar. The absorption coefficient for the nitrogenatedcarbon coated disc reaches a maximum of about 0.9 at about 450 nm andreaches a minimum of about 0.55 at 900 nm. The absorption coefficientfor the hydro-nitrogenated carbon coated disc reaches a maximum of about0.9 at about 475 nm and reaches a minimum of about 0.6 at 900 nm. Thelower absorption coefficient for the hydrogenated carbon coated discindicated that a hydrogenated carbon overcoat is preferred formagneto-optical discs over either a nitrogenated carbon overcoat and ahydro-nitrogenated carbon overcoat.

[0075] To evaluate the improved durability as a result of having acarbon overcoat, Contact-Start-Stop tests were performed onmagneto-optical discs using a model T1000 CSS tester from TTi, Milpitas,Calif. During one cycle of the test, a slider starts in contact with thesurface of a nonrotating disc. The disc was then rotated to a selectedmaximum speed and held at that speed for a selected number of secondbefore the disc was decelerated and stopped. After being stopped for ashort period, the cycle was repeated. The disc was examined for damageto the disc surface using an optical microscope with a 2000×magnification.

[0076] Five magneto-optical discs with different hydrogenated carboncoating thicknesses were tested. The disc had a 23Å lubricant layer ofZ-dol with a molecular weight from 2000 to 4000. The first disc had alayered structure as shown in FIG. 5 except that is lacked layer 186,the carbon layer. The disc had a 800 Å silicon nitride overcoat layer.The remaining four discs included a hydrogenated carbon layer betweenthe silicon nitride and the lubricant layers. Four different carbonlayer thicknesses were measured.

[0077] If the magneto-optical disc does not include a carbon coating, itcannot survive even on CSS test cycle. The inclusion of a even a 25 Åcarbon coating eliminates damage to the disc for greater than 20,000 CSScycles. Thicker hydrogenated carbon coatings also produced discs thatsurvived for greater than 20,000 CSS cycles. Thus, the inclusion of acarbon layer provides dramatic improvement in the durability of themagneto-optical discs. This improved durability allows for the use ofreduced fly heights in magneto-optical disc drives without resulting inunacceptably short disc lifetimes. Other observations of the carboncoated discs with a lubricant coating indicate that the discs haveacceptable values of stiction with read/write heads.

[0078] Although the present invention has been described with referenceto preferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A magneto-optical storage medium comprising: adata storage means for the optical encoding of data; and protectionmeans for improving the durability of the magneto-optical storage media.2. A magneto-optical storage medium comprising: a non-magneticsubstrate; a magnetic layer over the non-magnetic substrate, themagnetic layer comprising a magnetic metal or alloy having a Curietemperature accessible by optical heating; a carbon layer over themagnetic layer.
 3. The storage medium of claim 2 wherein the carbonlayer comprises hydrogenated carbon.
 4. The storage medium of claim 2wherein the carbon layer has a thickness greater than about 25 Å.
 5. Thestorage medium of claim 2 wherein the carbon layer has a thicknessgreater than about 30 Å.
 6. The storage medium of claim 2 wherein thecarbon layer has a thickness from about 35 Å to about 40 Å.
 7. Thestorage medium of claim 2 wherein the carbon coating has an absorptioncoefficient less than about 0.3 at a selected wavelength between about190 nm and about 900 nm.
 8. The storage medium of claim 2 wherein thecarbon coating has an absorption coefficient less than about 0.1 at aselected wavelength between about 190 nm and about 900 nm.
 9. Thestorage medium of claim 2 further comprising a lubricant layer.
 10. Thestorage medium of claim 9 wherein the lubricant layer comprises aperfluoropolyether.
 11. The storage medium of claim 9 wherein thelubricant layer has a thickness less than about 35 Å.
 12. The storagemedium of claim 9 wherein the lubricant layer has a thickness from about15 Å to about 25 Å.
 13. The storage medium of claim 1 further comprisinga dielectric layer between the magnetic layer and the carbon layer. 14.A cartridge comprising a magneto-optical disc of claim 2 within a coverthat provides selectable access to the disc.
 15. A disc drive comprisinga magneto-optical disc of claim 2 and an actuator connected to an armthat suspends a head in the vicinity of the surface of the disc.
 16. Amethod of producing a magneto-optical disc comprising depositing acarbon layer onto a disc with a magnetic layer comprising a magneticmetal or alloy having a Curie temperature accessible by optical heating.17. The method of claim 16 wherein the carbon layer is deposited bysputtering.
 18. The method of claim 17 wherein the sputtering isperformed in the presence of hydrogen.
 19. The method of claim 16further comprising applying a lubricant layer over the carbon layer. 20.The method of claim 19 wherein the carbon layer has a thickness greaterthan about 25 Å.